Flow soldering is a technology for soldering electronic components onto an electronic circuit substrate. An overview of this is explained below. Firstly, the electronic components are mounted on one main surface of the electronic circuit substrate. The leads of the mounted electronic component are inserted into through holes in the electronic circuit substrate and project out from the other main surface of the electronic circuit substrate. Molten solder makes contact with the surface where the leads project, thereby soldering the electronic component to the electronic circuit substrate. Below, one of main surfaces where the electronic component is mounted is called the component surface and the other one of main surfaces with which molten solder makes contact is called the soldering surface.
A flow soldering apparatus comprises processing devices, such as a fluxer, a pre-heater, a jet solder tank, a cooler, and the like. The fluxer, pre-heater, jet solder tank and cooler are disposed in this order facing the direction of conveyance of the electronic circuit substrate. The electronic circuit substrate is introduced into this flow soldering apparatus. As described above, an electronic component is mounted previously on the electronic circuit substrate which is introduced into the apparatus, and the leads of the mounted electronic component project from the soldering surface. The electronic circuit substrate introduced into the apparatus is conveyed by a conveyor which is disposed in the flow soldering apparatus. When the electronic circuit substrate is introduced into the flow soldering apparatus, firstly, flux is applied by the fluxer to the soldering surface of the electronic circuit substrate that has been introduced. Thereupon, the electronic circuit substrate onto which flux has been applied is preheated by the pre-heater. Next, molten solder at a high temperature is applied by the jet solder tank to the soldering surface of the electronic circuit substrate which has been preheated. Thereupon, the electronic circuit substrate to which the molten solder has been applied is cooled by the cooler. By means of these steps, an electronic component is soldered onto the electronic circuit substrate.
As described above, flux is applied to the soldering surface from which the leads project, whereupon the molten solder makes contact with the soldering surface. Flux is applied in order to remove oxide film and dirt which may adhere to the soldering surface. In general, liquid flux is used in soldering. Liquid flux includes a solvent and an active material, such as rosin, which is dissolved in this solvent. The fluxer which applies the liquid flux to the soldering surface of the electronic circuit substrate may be a foam fluxer, spray fluxer, or the like. The pre-heater preheats the flux that has been applied to the soldering surface, to a temperature of 100 to 150 degrees C. Due to this preheating, the active material performs its action and cleans the soldering surface of the electronic circuit substrate. If preheating is not carried out, then the soldering surface is not cleaned. Furthermore, a solvent for dissolving the active material is used in the liquid flux. If this solvent remains on the soldering surface of the electronic circuit substrate, then soldering of good quality cannot be achieved. In order to remove solvent, it is necessary to carry out preheating by means of a preheater.
The active material, such as rosin, dissolves well in alcohols, such as isopropyl alcohol. Therefore, an alcohol has been used in the solvent of the flux. However, if a volatile organic compound (VOC) such as alcohol escapes into the atmosphere, then the volatile organic compound is decomposed by ultraviolet energy, or the like, and creates radicals. These radicals are a cause of photochemical smog, or the like. Therefore, in Japan, the use of VOC is to be restricted from the year 2010, due to the amended Air Pollution Control Act. These restrictions will also apply to the field of soldering. For reasons of this kind, liquid VOC-free flux which does not use VOC as a solvent, and liquid low-VOC flux which has a reduced content of VOC have been developed. Low-VOC flux generally contains not more than 5 wt % of VOC.
If liquid flux which uses an alcohol having a low boiling point as the solvent is applied to the soldering surface, then the solvent evaporates if preheating is carried out at a temperature of 100 to 150 degrees C. On the other hand, VOC-free flux and low-VOC flux which have been developed recently use water which is less volatile than alcohols as the solvent. The water used as a solvent is difficult to remove completely before the molten solder makes contact with the electronic circuit substrate, at a preheating temperature of 100 to 150 degrees C. Consequently, when molten solder having a high temperature is applied by the jet solder tank onto the electronic circuit substrate onto which the VOC-free flux or low-VOC flux has been coated, problems arise, such as the occurrence of solder balls, solder wetting defects, solder bridging, or the like. Consequently, if VOC-free flux or low-VOC flux is applied, then it has not been possible to obtain a finish equivalent to that obtained when VOC flux which uses an alcohol as the solvent is applied. For reasons of this kind, technology has been proposed for eliminating the water content from the electronic circuit substrate before the molten solder is applied to the electronic circuit substrate by the jet solder tank.
For example, Patent Literature 1 discloses a flow soldering apparatus which comprises a preheating device disposed before a fluxer. This flow soldering apparatus preheats the electronic circuit substrate to a temperature of 100 to 200 degrees C. by means of the preheating device, and applies the liquid flux from the fluxer onto the preheated electronic circuit substrate. With this flow soldering apparatus, the water content in the applied flux is evaporated off by the heat of the electronic circuit substrate.
Patent Literature 2 discloses a flow soldering apparatus which comprises a hot air drying device provided between a fluxer and a jet solder tank. This flow soldering apparatus evaporates the water content included in the applied flux by directing hot air onto the electronic circuit substrate to which liquid flux has been applied.
Incidentally, it is difficult to remove completely the water content in the through holes of the electronic circuit substrate, and even if the surface of the electronic circuit substrate is dried, moisture remains in the through holes. Therefore, even when the surface of the electronic circuit substrate is dried, if there is a large amount of water in the through holes, then the water content in the through holes is converted into water vapor by the heat of the molten solder, and the molten solder is scattered about by this water vapor. However, until now, the preheating profiles of flow soldering apparatuses have not been set by taking account of the moisture in the through holes. Consequently, with the flow soldering apparatuses described above, there is a risk that soldering defects may occur.
Furthermore, there is variation in the water content contained previously in the respective electronic circuit substrates which are introduced into a flow soldering apparatus, and there is also variation in the state of application of the flux by the fluxer. The flow soldering apparatuses described above are not able to determine the amount of water contained in the electronic circuit substrate which is the work object. Hence, there is a risk that due to variations of this kind, an electronic circuit substrate from which the water content has not been removed sufficiently may be conveyed into the jet solder tank. If the water content is not removed sufficiently, then the molten solder is scattered. Consequently, with the flow soldering apparatuses described above, there is a risk that soldering defects may occur.